Patients with a deficiency of either LPL or apoC-II present with familial chylomicronemia, a rare genetic disorder characterized by severe hypertriglyceridemia, chylomicronemia and increased risk of pancreatitis. In order to gain insight into the structure-function relationship of these two proteins we are studying the underlying molecular defects in the LPL and apoC-ll genes of patients with familial hyperchylomicronemia. The gene defects in two such patients have been elucidated. The proband from the first kindred is a 58 yo black male with a functional deficiency of postheparin plasma LPL. Sequence analysis of patient PCR amplified DNA identified two separate mutations in each LPL allele. Thus, the patient was shown to be a true homozygote for two single base mutations resulting in an Asp-9 to Asn and Tyr-262 to His substitutions. Transient expression of LPL containing both mutations or substitutions in either Asp-9 or Tyr-262 resulted in the secretion of fully functional (LPL-9) and inactive (LPL-262, LPL-9,262) enzymes. All three expressed mutant enzymes retained normal heparin and lipid affinities. Analysis of these native mutations and their ability to disrupt some but not all LPL functions provides insight into important structural domains necessary for specific LPL function. We have previously described the underlying molecular defects in the apoC-ll and apoE genes of a unique patient with a combined functional deficiency of both apolipoproteins. The proband, a 41 yo white female from Decaze, France, presented with classical clinical features of familial chylomicronemia. Two independent mutations have been identified that lead to the synthesis of a 231 aa truncated apoE and the substitution of Phe-67 by Ser in apoC-ll. We have now generated a plasmid which can be used to express a fully functional apoC-ll in human embryonal kidney-293 cells. By using this expression system, we have shown that the expressed mutant apoC-ll (Phe-67 to Ser) is fully active. Therefore, the hyperchylomicronemia in this patient is not a result of a defective apoC-ll. In the presence of normal LPL activity, additional studies will be required to understand this patient's unique clinical presentation.